There are many circumstances in which it is desirable to print invisible indicia or areas on a paper substrate, and then make those areas visible in a simple and easy manner. Such a need exists in the production of educational forms, but is particularly useful in the security document area. Especially with the introduction of modern color copiers, amateur and professional counterfeiters can reproduce all sorts of valuable documents, such as admission tickets, passes, coupons, certificates, bank checks, lottery tickets, money orders, and the like. Many businesses and agencies suffer substantial losses as a result of the production and distribution of such bogus documents, yet most cannot justify the cost of security devices that are conventionally available to protect against this type of fraud.
According to the present invention, a method and product are provided in which essentially invisible indicia or spots are easily applied, utilizing conventional equipment, to a paper substrate. By using a simple conventional highlighting marker, or the like, one can easily test the document for authenticity, or otherwise utilize the document, by making the hidden indicia clearly visible. Since the invention utilizes very simple components, conventional equipment, and simple techniques, it can produce a security document such as a ticket, bank check, stock certificate, coupon, or the like, at almost no increase in cost compared to a non-protected document.
The basic aspect of the present invention is the utilization of a clear toner which can be electrostatically imaged onto a piece of paper, as by using a dual roller fluidized electrostatic bed applicator. The toner, which is simple to construct, is initially applied to the paper, in its virgin form, as a snow white deposit, which has a clear contrast with the surface of the paper (even if itself white). However then when the toner is heated to fuse it, as by a conventional infra-red heat fuser, it melts into the fibers of the paper and turns clear so that it is essentially invisible to the naked eye when illuminated by any wavelength of light (unless a particular dye is added to it to intentionally make it visible when illuminated by infra-red or UV light). The applied toner is hydrophobic, so that when a water base low concentrate ink, such as in a colored felt tip marker, or the like, is applied to the paper substrate at the area of the imaged toner, the toner itself repels the marker, but the surrounding paper readily absorbs it, so that the toner is clearly visible as a "negative" image. This allows one, for example a ski lift operator, to easily and simply check the authenticity of presented documents, such as ski lift tickets. The person checking the ski lift ticket need only use a conventional marker, and know where to mark the presented ticket to render visible the security indicia. The clear security indicia cannot be reproduced successfully even by a modern color copier.
According to one aspect of the present invention, a method of providing and using security indicia on a paper substrate utilizing a substantially clear hydrophobic toner is provided, the method comprises the following steps: (a) Charging and applying the toner in the form of security indicia to the paper substrate. (b) Heat fusing the toner so that it blends into the paper and becomes substantially invisible to the naked eye and color copiers regardless of the wavelength of light directed onto it. And, (c) when it is desired to view the indicia, applying an aqueous based solvent system having a distinctly different color than the paper substrate to the paper substrate over the toner, the paper surrounding the toner indicia absorbing the solvent while the toner indicia does not, making the toner indicia visible.
Step (a) is practiced by imaging the toner onto the paper electrostatically, e.g. using ionographic, laser, and ion deposition imaging techniques, and step (b) is typically practiced by infra-red heat fusing. Step (a) is further practiced by using a polyester resin based toner which has been milled and classified so that the particle size is very uniform, and mixed with a silica flowing agent. The toner may be variably imaged on the paper, and also visible indicia corresponding to the invisible indicia may also be variably imaged onto the paper. Alternatively, it can be overprinted, or overcoated, for example by applying foil over it. If it is desirable to make the indicia visible in certain light wavelengths, a UV responsive dye or an infra-red absorbing dye may be added to the toner. The invention also contemplates a security document made by the method described above.
According to another aspect of the present invention, a security document is provided comprising: A substantially paper substrate; and indicia imaged on the substrate comprising a hydrophobic toner blended into the paper substrate so that it is substantially invisible to the naked eye and color copiers when illuminated with ambient light.
The invention also comprises another type of security document, particularly useful in making lotto tickets or the like which has a different receptivity to overprinted inks and coatings for rub off or image permanence considerations. The security document according to this aspect of the invention comprises: A paper substrate. A spot of hydrophobic toner imaged onto and blended into the paper substrate so that it is substantially invisible to the naked eye and color copiers when illuminated with ambient light. And, an overcovering or overprinting on the spot.
The invention also contemplates a method of treating a substrate comprising the following steps: (a) Producing a clear toner by milling a polyester resin to an average particle size of about 15 microns, and classifying the resin to reduce particles of less than about 5 micron to under about 10% of the total distribution of particles, and blending the milled and classified particles with a silica flowing agent, and intimately mixing them together. (b) Electrostatically imaging the toner onto a paper substrate. And, (c) heat fusing the toner, so that it blends into the paper substrate and becomes substantially invisible to the naked eye and color copiers when illuminated with ambient light. Step (b) is typically practiced by imaging the toner on the surface as human or machine readable indicia, and there is typically the further step of imaging visible toner on the substrate corresponding to the invisible toner imaged thereon by the practice of step (b).
It is the primary object of the present invention to provide a simple and effective technique for producing documents having invisible indicia or areas which are readily rendered visible, particularly for security document applications. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.